This invention refers to dental retention pins comprising a first section, which is to be inserted or screwed into the tooth, and a second section, which extends from the tooth into the space for the filling structure and which improves the retention to the tooth.
Dental retention pins are used for the restoration of teeth in order to obtain a main or an additional retention for the filling structure within the remainder of the tooth structure, which is called ivory. A pin is inserted, preferably screwed into a borehole of the tooth with one section, whereas the other section extends from the tooth and is embedded into the filling material around the pin. Metal pins result in a mechanical joint with the filling structure; therefore that part of the pin which is embedded into the filling structure for improving said mechanical joint with the filling material is provided with screw-like or any other positive or negative macroscopic deformations so that the filling material, especially under tension and compression forces, cannot separate from the pin. Because the filling material under chewing conditions undergoes considerable stresses, such as tension and compression forces, this type of mechanical joint, which does not allow an exact body contact between pin and filling structure, in general is not sufficient. Between the filling material and the pin an edge gap is formed. Locations at which the pin does not have a firm and complete contact with the filling material over the full surface are positions of weakness, which do not add to the stablility and durability of the filling (or alternatively, the joint between the filling and the tooth) and result in fast aging of the plastics material or the filling material in these areas. This which decreases the retention of the pin continuously with increasing time. Therefore, this method, which up to now is used extensively, is not satisfactory. As filling materials nowadays preferably amalgams, plastics filling materials and glass ionomer cements are used. A chemical bond, which would overcome these disadvantages, up to now only can be obtained if pins plated with gold or silver are used and if amalgam is used as a filling material. However, because amalgam fillings in the field of teeth restoration are more and more being replaced by other types of filling materials, because plastics materials as well glass ionomer cements (in addition to the main field of use within the front teeth area) are being used in the lateral teeth area, the forementioned method is of decreasing importance. This is so because the joint between a retention pin made of metal or other material and the filling mass made from platics material or from glass ionomer cement or from other non-metallic materials generally results in an exclusively mechanical joint.
A mechanical joint has considerable disadvantages compared with chemical or alternatively physical-chemical bondings, because with a mechanical joint micro-edge-gaps are obtained, which allow very small relative movements between pin and filling material. Because liquid and bacteria are able to enter such gaps they cause corrosions and aging of the filling materials, as well the danger of secondary caries. Furthermore, it is to be expected that the filling materials will cause coloration and that the dark metal pin will shine through. Finally and especially, the retention power of a pin with a mechanical joint to the filling material is lower by far than a chemical bond or a physical-chemical bond under the wet conditions within the mouth, and will decrease with continuous aging of the plastics material, as detailed investigations of the inventor have shown. This problem is still more serious if such pins will be used for teeth-superstructures and if structure material is to be ground and insufficient thickness of material will be left in order to support the pin and to provide a correct joint.
The problem of micro-edge-gaps within the compound plastics/metal has been known for a long time in the dental field. One solution for this problem is the use of an inorganic intermediate layer as a bond assisting layer, which results in a chemical bond to the substrate and to the plastic material. On a metal surface, which preferably is treated by sandblasting, an SiO.sub.x -C layer, on top of it a silane bond assisting layer, and on top thereof a plastics layer, are applied. The bond assisting layer provides the chemical bond between the plastics material and the SiO.sub.x C layer. This type of plastics/metal-compound in the dental field up to now has been used exclusively for larger, substantially plane surfaces, namely in connection with crown structures and bridge structures within the crown area, in order to face a metal surface with plastics material. The bonding surface always has a relatively large surface area, and the plastics material is the outermost, which means the visible side of the completed tooth. This plastics material also will have to meet the requirements for high surface quality (amenability to receive polish, density, etc.).